(a) Field of the Invention
The present invention relates to an electrostatic ink jet printer and, more particularly, to a head drive unit in an electrostatic ink jet printer, which controls the dot diameter of an ink droplet made of colored particles ejected from pigment ink.
(b) Description of the Related Art
Electrostatic ink jet printers are increasingly used for a personal computer due to its high printing performance as well as small noise. A printing head and a head drive unit in a conventional electrostatic ink jet printer will be described first with reference to FIGS. 1 to 4.
FIG. 1 is a perspective view of a printing head 100 in the conventional ink jet printer, FIG. 2 is a schematic block diagram of the printing head 100 of FIG. 1 and an associated head drive unit 200, FIG. 3 is a timing chart of signals applied to an electrophoretic electrode and an ejection electrode in the printing head 100, and FIG. 4 is a timing chart of ejecting pulses having an ejecting voltage Vej and supplied to ejection electrodes for recording different dot diameters.
In FIG. 1, the printing head 100 includes an ink chamber 102 receiving therein pigment ink 101 and having an ink ejection slit 104 at the front edge thereof, a plurality of ejection electrodes 106 extending in parallel to one another from the rear edge to the front edge of the printing head 100, an electrophoretic electrode 103 disposed at the rear edge of the ink chamber 102 for driving colored particles in the pigment ink 101 toward the ink ejecting slit 104 for concentration of the colored particles at the ink ejecting slit 104, and a counter electrode 107 disposed on the back surface of a recording sheet 105 to oppose the front tips of the ejection electrodes 106.
The ink ejecting slit 104 is partitioned by passage walls 108 corresponding to respective ejection electrodes 106 to generate an ink meniscus of pigment ink on each ejection electrode 106. The ink chamber 102 is communicated with an ink reservoir (not shown) at an ink inlet port 109 and an ink outlet port 110 through ink tubes. Thus, a back pressure is applied to the pigment ink in the ink chamber 102, and the pigment ink 101 in the ink chamber 102 is forced to circulate between the ink chamber 102 and the ink reservoir.
The head drive unit 200, as shown in FIG. 2, has an image data control section 205 for receiving gray-scale image data from a processor, a driver control section 203 for generating switching signals for controlling switches in a driver section 202 based on the image data, a pulse width generator 204 for generating pulse width signals based on the image data, and the driver section 202 including a plurality of switches each for receiving the switching signal from the driver control section 203 to apply an ejecting voltage Vej to a corresponding ejection electrode 106 during a time interval based on the pulse width signals.
In operation, the printing head 100 uses an electrophoretic phenomenon wherein colored particles in the pigment ink 101 are driven in a direction specified by an electric field applied to the pigment ink 101 containing electrified colored particles. More specifically, when a constant electrophoretic voltage V1 shown in FIG. 3 is applied to the electrophoretic electrode 103 to generate an electric field in the ink chamber 102 filled with the pigment ink 101, colored particles in the pigment ink 101 move toward the ink ejecting slit 104 at an electrophoretic speed.
After the colored particles move toward the ink ejecting slit 104, an ink meniscus 206 is form at the tip of each ejection electrode 106. When a switch in the driver section 202 shown in FIG. 2 is turned on, an ejecting pulse having a constant voltage Vej and a duty ratio of 50 to 100%, as shown in FIG. 3, is applied to a corresponding ejection electrode 106. Thus, colored particles are driven by the electrostatic field generated between the ejection electrode 106 and the counter electrode 107, and ejected from the ink ejecting slit 104 against the surface tension of the ink meniscus 206 and the viscous force of the pigment ink 101. The colored particles are ejected as ink droplets 201 from the tip of the ejection electrode 106 in synchrony with the ejecting pulse PEJ to adhere onto the recording sheet 105 as a dot. The colored particles are replenished from the ink reservoir to be iteratively ejected to form an image on the recording sheet 105.
A conventional technique for forming a desired dot diameter based on the level of the gray-scale data will be now described.
For obtaining a desired dot diameter of the ink droplet 201, correlation between the dot diameter and the pulse width of the ejecting pulse such as shown in FIG. 3 is experimentally determined and the list of the pulse widths is stored in combination with the level of the gray-scale image data in a storage device or a memory. The image data control section 205 receives gray-scale image data from the processor, retrieves a pulse width corresponding to the level of the gray-scale image data in the storage device, and transmits the pulse width data to the pulse width generator 204. The image data control section 205 also transmits the image data for controlling on/off of the switch in the driver section 202 to the driver control unit 203. The pulse width generator 204, after receiving the pulse width data, generates a pulse width signal based on each gray-scale level of the ejection electrodes 106 to supply ejecting voltage Vej. Thus, the driver control section 203 closes the switches in the driver section 202 during time intervals based on the respective gray-scale image data to thereby apply the ejecting voltage Vej to the ejection electrodes 106.
In the example of FIG. 4, it is assumed that forty ejection electrodes 106-1 to 106-40 are provided in the ink jet printer and are applied with the depicted ejecting pulses. If the ejection electrodes 106-1, 106-2, 106-3 and 106-40 are desired to form dot diameters of 20 xcexcm, 50 xcexcm, 75 xcexcm, and 100 xcexcm, respectively, ejecting pulses PEJ having pulse widths of 50 xcexcs, 80 xcexcs, 90 xcexcs and 100 xcexcs are applied to the ejection electrodes 106-1, 106-2, 106-3 and 106-40, respectively. The respective pulse widths provide desired dot diameters of the ink droplets based on the gray-scale image data, thereby forming desired image data on the recording sheet 105.
The conventional ink jet recording device as described above has a disadvantage in that the circuit scale of the pulse width generator 204 increases with the increase of the number of ejection electrodes 106 provided and the number of gray-scale levels supplied.
In addition, when a plurality of ejection electrodes 106 have variations of the electric resistance therealong, the dot diameters formed by the respective ejection electrodes 106 depend on the variations of the electric resistance, thereby degrading the printing quality for the gray-scale level.
It is therefore an object of the present invention to provide an ink jet printer including a head drive unit having a simpler structure of the pulse width generator even if the number of ejection electrodes and the numbers of gray-scale levels increase.
It is another object of the present invention to provide a uniform dot diameter without depending on variations of the electric resistance of the ejection electrodes.
The present invention provides an ink jet printer comprising a printing head including an ink chamber for receiving therein pigment ink, the ink chamber having an ink jet slit, and an array of ink ejection electrodes, disposed in the ink chamber, for receiving an ejecting voltage to eject the pigment ink from the ink jet slit, and a head drive unit for receiving a set of recording data for the ejection electrodes during each recording clock cycle to generate a plurality of sets of first data during each recording clock cycle based on the recording data, each set of the first data including a bit data for each of the ejection electrodes, a combination of the bit data for each of the ejection electrodes in each recording clock cycle specifying a pulse width of the ejecting voltage for the each of the ejection electrodes.
In accordance with the ink jet printer of the present invention, head control section can provide a pulse width of the ejecting voltage for each ejection electrode based on the combination of bit data, thereby generating the pulse width data with a simple structure. Further, a pulse width can be selected to cancel the variations of electric resistance of ejection electrodes.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.